The invention relates to an apparatus for the production of an expanded dry feed or foodstuff from proteinaceous and possibly starchy raw materials or screw extruder, in which raw materials are mixed with water and extruded at elevated temperatures and pressures into a material which on issuing from the extruder spontaneously expands under loss of moisture.
In a known process the raw materials are mixed with water until a moisture content of 25 - 35 percent by weight is reached, and then subjected to the action of an extruder-cooker in which the mixed materials are converted into a fluid mass under the influence of heat and high pressure. The temperature of this mass lies above 100.degree. C. When this mass is pressed through the extruder orifice into an environment with atmospheric pressure, the water heated to above its boiling point will escape more or less explosion-like as steam, because of which the mass will expand into a cellular material. This material is subsequently cut into bits and further dried until it reaches a stable moisture content of about 10 percent by weight. The disadvantage of this process is that the product issuing from the extruder is still too wet and should be dried before it is ready for shipment. This drying step increases the cost of the product and causes important losses because of dust formation, disintegration and the like.
Thus it was suggested in the prior art to obviate this drying step by abstaining from adding additional moisture to the starting products with a balanced moisture content, but adding instead an organic edible solvent, which has in general a low volatility, like glycerol and propylene glycol.
Such a process, however, provides a product in which a large part of the added organic liquid remains. Because of this the texture of the obtained product will be permanently affected, which is hardly desirable for some applications.
Thus the desire remains to provide a process in which the extruded product needs no additional drying step, too, but in which no foreign materials should remain in the product.